1. Field of the Invention
The present invention relates to inductors for use in high frequency integrated circuits.
2. Description of the Related Art
Series resistance is inherent within inductive structures. Series resistance within inductive structures formed by a silicon process dominates the losses occurring during operation as the frequency of operation increases. The losses reduce the inductor's quality factor Q, the ratio of reactance to series resistance within the inductor (when the inductive structure is modeled using a certain topology). Reducing or minimizing the increasing series resistance with increasing frequency, with its concomitant effect on the inductor's Q, is accomplished by increasing the cross-sectional area for current flow within the inductor. Increasing the cross-sectional area may be accomplished by increasing the metallization width or thickness, or both, of the conductive path forming the inductor.
An improved Q displayed by an inductor as a function of increased width W or depth D is substantially linear at DC to the lower frequencies. As the frequency of operation increases, however, current flow through the entire cross-sectional area of the inductor's conductive path, tends to drop off. The current thereafter tends to flow at the outer cross-sectional edges (i.e., perimeters) of the cross-section of the inductor, such as L10 depicted in FIG. 1A. Such current flow is in accordance with the so-called "skin-effect" theory.
Inductors formed for use within integrated circuits are typically spiral-shaped. FIG. 1B shows a portion of a conventional spiral inductor, L20, formed with an aluminum conductor 24 on a silicon substrate 22. FIG. 1C shows a cross-sectional portion of the conductive path of conductor 24. W and L represent the conductor's width and length, respectively, and D represents its depth. L is the summation of individual lengths 1.sub.1, 1.sub.2. . . 1.sub.n, comprising the inductor's conductive path. Because the conductive path is spiral-shaped (although not clear from the cross-sectional view in the figure), magnetic fields induced by current flow tend to force the current to flow along the inner or shorter edges of the spiral conductive path (shown hatched). Because of these "edge effects", increasing the width W beyond a particular point (and therefore the cross-sectional area), as mentioned above, ceases to show a concomitant improvement in the inductor's Q with increasing frequency. The thickness or depth D of the conductive path must be increased, or the magnetic coupling between adjacent turns must be increased, to provide the required Q.